1. Field of the Invention
The present invention relates to proteins that are members of the IL-1 receptor family. More particularly, the present invention relates to IL-1Rrp1 and AcPL receptor complexes that mediate high affinity IL-18 binding and activity as well as inhibit IL-18 mediated activity.
2. Description of Related Art
The type I interleukin-1 receptor (IL-1RI) mediates the biological effects of interleukin-1, a pro-inflammatory cytokine (Sims et al., Science 241:585-589, 1988; Curtis et al., Proc. Natl. Acad. Sci. USA 86:3045-3049, 1989). A second interleukin-1 receptor (designated type II IL-1R or IL-1RII) binds IL-1, but does not appear to mediate signal transduction (McMahan et al., EMBO J. 10:2821, 1991; Sims et al., Proc. Natl. Acad. Sci. USA 90:6155-6159, 1993). IL-1RI and IL-1RII each bind IL-1 and IL-1β. IL-1 has been implicated in chronic inflammatory diseases, such as rheumatoid arthritis and inflammatory bowel disease. There is increasing evidence that IL-1 plays a role in osteoporosis. All of these activities are initiated by the signaling function of the cytoplasmic portion of the Type I IL-1R. IL-1ra inhibits the activities of IL-1 by binding to the type I IL-1 receptor, thereby blocking access to IL-1α and IL-1β while eliciting no biological response of its own.
IL-1RI and IL-1RII belong to a family of proteins that exhibit significant sequence homology. One such protein is IL-1R accessory protein (IL-1R AcP), described in Greenfeder et al. (J. Biol. Chem. 270: 13757-13765, 1995). This protein, by itself, is not capable of binding IL-1, but does form a complex with IL-1RI and IL-1α and IL-1β. When co-expressed with IL-1RI, recombinant IL-1R AcP increases the binding affinity of IL-1RI for IL-1β (Greenfeder et al., supra).
Another protein exhibiting sequence homology to the IL-1RI and IL-1RII family is the IL-1 receptor related protein 1 (IL-1Rrp1) (See Parnet et al. J. Biol Chem 271:3967, 1996, and Torigoe et al., J. Biol Chem 272:25737, 1997). Still another such protein is AcPL.
IL-18 is a homologue of IL-1α and IL-1β and is known to activate many of the same responses activated by IL-1. For example, cells stimulated with IL-18 activate NFκB and produce known inflammatory mediators. IL-18 acts as a stimulator of Th1 cell growth and differentiation and is a potent inducer of γ-interferon production from Th1 cells. The Th1 class of helper T cells are known to mediate inflammatory reactions. IL-18 enhances NK cell killing activity and has been implicated in septic shock, liver destruction, inflammatory bowel disease and diabetes.
Recently it was shown that IL-1Rrp1 binds IL-18 and mediates IL-18 signaling in transfected cells. However, the IL-1Rrp1 binding affinity for IL-18 is very low and it is likely that one or more additional receptors or receptor subunits are involved with IL-18 binding and signaling.
Thus, the identification of additional receptors of for IL-18 is desirable. Such receptor proteins can be studied to determine whether or not they bind IL-18 and, if so, whether the receptors play a role in mediating signal transduction. Furthermore, soluble forms of such receptors may be used to inhibit IL-18 activity and ameliorate any inflammatory and/or autoimmune diseases attributable to IL-18 signaling. The possible existence of additional affinity-converting subunits for IL-18 can be explored, as well.